The present invention relates generally to electrical stimulators for biological tissue and more particularly to an electrical current output circuit for an electrical stimulator for biological tissue.
Electrical stimulators providing an electrical stimulus signal are useful exciting for biological tissue. One significant use for electrical stimulators of this type is for transcutaneous electrical nerve stimulation (TENS) in which carefully controlled electrical stimulus signals are generated and then delivered via suitable electrodes through a patient's skin to underlying biological tissue. The electrical stimulus signals can be utilized for the purpose of masking neurally conducted pain signals; for example, the sensation of pain felt by a patient after surgery. Because of a patient's response to transcutaneous electrical nerve stimulation may vary significantly, a wide range of electrical stimulus parameters must be provided. A second use of electrical stimulators is for neuromuscular stimulation (NMS) in order to initiate or control muscular action in a patient. Since a wide variety of muscular actions are available, a wide variety of electrical stimulus signals must again be provided.
Electrical stimulators of biological tissue are typically attached to the biological tissue with electrodes. It is generally desirable that the electrode-tissue interface be low in impedance. If the impedance of the electrode-tissue interface is low, then most of the power supplied by the electrical stimulator will reach the biological tissue. However, the electrode-tissue interface sometimes exhibits a high impedance. If the electrode-tissue interface develops a high impedance, then much of the power provided by the electrical stimulus signal will be dissipated at the electrode-tissue interface instead of in the intended biological tissue. Such power dissipation at the electrode-tissue interface may cause skin irritation and other deleterious side effects. Moreover, this condition may lead to insufficient stimulation of the biological tissue unless extremely high voltages are provided by the electrical stimulator. For these reasons, some electrical stimulators for biological tissue utilize an impedance monitoring system to attempt to detect the condition of a high electrode-tissue interface impedance.
Prior art circuits have been utilized to sense for a high electrode-tissue impedance.
With a transformer driven output, one typical circuit has been constructed which utilizes an extra winding on the primary side of the output transformer output. If there is an extremely high load impedance across the secondary due to a poor electrode-tissue interface, the energy in the secondary will be reflected back to the primary winding and to the additional winding inserted on the primary side for this purpose and the energy in that winding is sensed. This solution adds to the cost, complexity and, size of the electrical stimulator due to the need for the additional transformer winding. Moreover, it is difficult to design an associated electronic circuit which will signal that a specific value of load impedance has been exceeded over a wide range of stimulus parameters and waveform types.
Another prior art circuit uses a series impedance element with the intended load. If there is little or no voltage across the series impedance element then the circuit knows that little or no current is flowing to the load and thus a high electrode-tissue interface impedance can be sensed. This circuit, however, is sensitive to ordinary variations and output current levels. In addition, sometimes the series impedance element is somewhat non-linear, i.e. may act as a rectifier. In this case, the series impedance element causes a DC shift in the electrical stimulus signal, resulting in polarization and possible deplating of the electrodes and in the occurrence of skin irritation.